There have been proposed a variety of pin junction photovoltaic elements for solar cells and for power sources in various electric appliances. Such photovoltaic elements are formed by ion implantation or thermal diffusion of an impurity into a single crystal substrate of silicon (Si) or gallium arsenide (GaAs), or by epitaxial growth of an impurity-doped layer on said single crystal substrate. However, there is a disadvantage for these photovoltaic elements that their production cost unavoidably becomes high because of using said specific single crystal substrate. Because of this, they have not yet gained general acceptance for use as solar cells or as power sources in electric appliances.
Recently, there has been proposed a photovoltaic element in which there is utilized a pin junction of amorphous silicon (hereinafter referred to as "A-Si") deposited film formed on an inexpensive non-single crystal substrate of glass, metal, ceramic or synthetic resin by way of glow discharge decomposition method. This photovoltaic element has a nearly satisfactory performance and is of low production cost and because of this, it has been recognized usable as a power source for some kinds of appliances such as electronic calculators and wrist watches.
However, for this photovoltaic element, there is a disadvantage that the output voltage is low because the band gap of the A-Si film constituting the element is about 1.7 eV, which is not large enough. There is another disadvantage that its photoelectric conversion efficiency is low for a light source such as fluorescent light which contains short-wavelength light in a dominant proportion, so that its application is limited to appliances with very small power consumption.
There is a further disadvantage for said photovoltaic element that the constituent A-Si film is often accompanied with a character of the so-called Staebler-Wronsk effect, with which the film being deteriorated upon continuous irradiation with intense light for a long period of time.
For a photovoltaic element to be immobilized as a power solar cell, it is necessary to convert efficiently and continuously the light energy of sunlight into the electric energy, and hence, it is desired to have such a layer structure that permits photoelectric conversion for sunlight over as broad a spectrum range as possible.
Now, in the case of a photovoltaic element which is made using a semiconductor material having a small energy band gap, the wavelength region of light to be absorbed by the layer is extended from the short wavelength side to the long wavelength side. However, in this case, it is the long-wavelength component of sunlight alone that contributes to photoelectric conversion, and the energy of the short-wavelength component is not served for photoelectric conversion. This is because the amount of energy to be outputted by the photoelectric conversion is decided upon the energy band gap of the semiconductor material as used.
On the other hand, in the case of a photovoltaic element which is made using a semiconductor material having a large energy band gap, the wavelength component which is absorbed by the layer and comes to contribute to photoelectric conversion is the short wavelength light having an energy exceeding the energy band gap of the semiconductor material as used, and the long-wavelength component is not served for photoelectric conversion.
By the way, in a photovoltaic element, the maximum voltage or open-circuit voltage (Voc) to be outputted is determined upon the energy band gap values of the semiconductor materials to be joined together. In view of this, in order to obtain a high Voc, semiconductor materials having a great energy band gap are desired to be used.
Therefore, there is eventually a limit for the photoelectric conversion efficiency for a photovoltaic element, which is prepared by using the sole semiconductor material.
The foregoing led to an idea of forming a plurality of photovoltaic elements using a plurality of semiconductor materials each having a different energy band gap, so that the individual photovoltaic elements become responsible for the different wavelength regions of sunlight. This idea was expected to contribute to an improvement in the photoelectric conversion efficiency.
However, there is a disadvantage for the solar cell having such layer structure as mentioned above that the high photoelectric conversion as a whole is possible only in the case where the individual photovoltaic elements have good characteristics, because it is of such structure that a plurality of photovoltaic elements are stacked to form an electrically serial structure.
Unfortunately, for the photovoltaic element having the foregoing structure, there has not yet realized any desirable one that the respective constitutent elements as stacked have satisfactory values of energy band gap and satisfactory characteristics as desired and that provides a high Voc as the photovoltaic element.
Besides, there have been proposed direct transition-type semiconductor films having a wide band gap, such as ZnSe (having a band gap of 2.67 eV) and ZnTe (having a band gap of 2.26 eV) and mixed crystal thereof ZnSe.sub.1-x Te.sub.x (where 0&lt;x&lt;1). And the public attention has been focused on these semiconductor films. These semiconductor films are, in general, such that are formed on a single crystal substrate by way of epitaxial growth. The as-grown film of ZnSe exhibits n-type conductivity and the as-grown film of ZnTe exhibits p-type conductivity. However for any of these films, it is generally recognized that it is difficult for the film to be controlled to the opposite conductivity. Further, in order to carry out the epitaxial growth upon the film formation, it is required to use a specific single crystal substrate and to maintain the substrate at elevated temperature. And in this film formation, the deposition rate is low. Because of this, it is impossible to perform epitaxial growth on a commercially available substrate which is inexpensive and low heat-resistant such as glass and synthetic resin. These factors make it difficult to develop practically applicable semiconductor films using the foregoing commercially available substrates.
Even in the case where a semiconductor film should be fortunately formed on such commercially available substrate, the film will be such that is usable only in very limited applications.
There have been various proposals to form a direct transition-type semiconductor film on a non-single crystal substrate such as glass, metal, ceramics and synthetic resin. However, under any of such proposals, it is difficult to obtain a desired direct transition-type semiconductor film having satisfactory electrical characteristics because the resulting film becomes to be accompanied with defects of various kinds which make the film poor in electrical characteristics and on account of this, it is difficult for the film to be controlled by doping it with an impurity.
In the meantime, amorphous film comprised of Zn and Se elements can be found in prior art references. As such prior art references, there are U.S. Pat. No. 4,217,374 (hereinafter, called "literature 1") and U.S. Pat. No. 4,226,898 (hereinafter, called "literature 2"). And ZnSe compound is described in Japanese Patent Laid-open No. 189649/1986 (hereinafter, called "literature 3") and Japanese Patent Laid-open No. 189650/1986 (hereinafter, called "literature 4").
Now, literature 1 discloses amorphous semiconductor films containing selenium (Se) or tellurium (Te), and zinc (Zn), hydrogen (H) and lithium (Li); but the subject lies in amorphous selenium semiconductor film or in amorphous tellurium semiconductor film, and the Zn described therein is merely an additive as well as Li and H. And as for the Zn and the Li, likewise in the case of the H, they are used aiming at reduction of the local state density in the energy band gap without changing the inherent characteristics of the film. In other words, the incorporation of Zn into the amorphous Se or the amorphous Te in literature 1 is not intended to positively form a ZnSe compound or a ZnTe compound. Incidentally, literature 1 mentions nothing about the formation of a ZnSe compound, ZnTe crystal grains or ZnSe.sub.1-x Te.sub.x crystal grains. And as for the addition of Li, it should be noted that it is not added as a dopant.
Literature 2 does mention amorphous semiconductor films containing Se or Te, and Zn, and H. However, it deals mainly with amorphous silicon, and it defines Se and Te as elements to form a compound with said silicon. As for the Zn, it defines as an element to sensitize the photoconductivity and reduce the local state density in the energy gap. In other words, the additions of Zn and Se are not intended to form a ZnSe compound, ZnTe compound or ZnSe.sub.1-x Te.sub.x compound. Incidentally, literature 2 mentions nothing about the formation of a ZnSe compound, ZnTe compound, ZnSe.sub.1-x Te.sub.x compound, ZnSe crystal grains, ZnTe crystal grains or ZnSe.sub.1-x Te.sub.x crystal grains.
Literature 3 and literature 4 are concerned with the deposition of a ZnSe film by HR-CVD method (hydrogen radical assisted CVD method). That is, they disclose methods of improving the deposition rate and the productivity of a deposited film; and they merely mention deposited films of non-doped ZnSe.
Against these backgrounds, there is an increased social demand to provide an inexpensive photovoltaic element having a high photoelectric conversion efficiency, particularly, for short-wavelength light which may be practically usable as solar cell and also as a power source in various electric appliances.